Hand and arm protection are critical elements in industrial safety. Gloves, sleeves, armbands, vests, coats, pants, leggings, and other protective garments are used to provide this protection. The materials from which they are constructed are fundamental to the level of protection they provide.
The introduction of advanced fiber technology such as para-aramid KEVLAR® brand fibers ushered in a new level of hand and arm and other bodily protection. Flexible and pliable, KEVLAR® brand fiber also provides extremely high strength and cut resistance. As a result, KEVLAR® brand fiber and other para-aramid fibers have been used in many protective items including string-knit gloves and sleeves, vests, coats, pants, leggings and other garments. Applicant makes no claim to the trademark KEVLAR®.
Special machinery designed for string knit processing has been developed over the past few decades, which provide a very low-cost method for producing finished string knit gloves and sleeves in particular. The products primarily provide a synthetic layer of skin over the human skin to provide protection against heat and cuts from sharp objects.
A major drawback with string knit products, however, is the open nature of the knit fabric. To provide the desired flexibility and lower cost, the fibers in the knit products tend to be spaced 1–3 mm (millimeters) apart. As a result, these knitted materials provide no protection against puncture or cut from objects smaller than the interstices of the knitted fabric. In industrial environments as well as the garden, many pointed objects including metal shavings, rose thorns, glass shards, and wood splinters are small enough to cause hand injury, even with the protection of a string knit glove. In addition, string knit structure is not optimized as a cut resistant substrate.
Other protective materials used in safety apparel include leather, rubber, and woven fabrics. However, most materials either cannot provide NPR greater than 50 ppi or are heavy and thick enough to render them unsuitable to the task. Glove liners with no significant puncture resistance have been used in combination with glove shells but are currently used to only improve comfort or thermal insulation.
Puncture resistance of a given material is typically determined as the force required to insert a specified penetrator through the material. Puncture resistance is typically reported as the peak force observed during the test. ASTM D1342 is useful as a gross indicator of puncture resistance, but does not adequately describe the overall material performance capabilities where puncture resistance as well as flexibility, dexterity and tactility are required. Warwick test method WTM-7N05 is used to characterize puncture resistance of materials. This test method is similar to ASTM D1342 with modifications to account for material thickness, field penetrator geometry and optical determination of test end point.
Thickness of a material is directly related to its flexibility and hence its comfort when installed in a product. Normalized puncture resistance (NPR), defined as puncture force divided by material thickness, is used to better differentiate relative puncture performance for a material class:       Normalized    ⁢                  ⁢    Puncture    ⁢                  ⁢    Resistance    ⁢                  ⁢          (      NPR      )        =                    Puncturc        ⁢                                  ⁢        Force                    Material        ⁢                                  ⁢        Thickness              .  
For example, the 2 lbf puncture resistance of material A is greater than the 1 lbf puncture resistance of material B. If, however, material A achieves that performance due to a significantly greater thickness, material A may in fact not be the optimum material for a given system. If material A has a thickness 5 times greater than material B, incorporating Material A into an article such as a glove would provide significant penalties for the user in terms of comfort and dexterity. Using the NPR rating of the material, if Material A were 0.10″, its normalized puncture resistance is 20 pounds per inch of thickness (20 ppi). By comparison, B would have a thickness of 0.02″, and its normalized puncture resistance would be 50 ppi. The normalized puncture resistance (NPR) rating better represents the benefits of material B relative to material A.
In many industrial, commercial and outdoor environments, puncture threats can be significantly smaller than the 0.080″ diameter penetrator used in ASTM D1342. Smaller penetrators require less force to pierce a substrate and are often the penetrator types to create failure in a protective system. These small penetrator types such as glass shards, wood splinters, thorns, and snake teeth can become deeply imbedded in a persons body and have the risk of serious infection. In other environments the threat can be the source of dangerous disease as is the case with contaminated fine gauge hypodermic needles. As a result, in the development of specialty PPE glove products, a test probe more representative of these critical threats is highly desirable. In WMT-7N05, a 0.050″ sewing needle has been substituted for the 0.080″ probe specified in ASTM D1342. Sewing needles are manufactured with reasonable hardness, sharpness and uniformity that make them suitable for use in a test method. They also have a more gradually tapered tip similar to many naturally occurring threats like glass shards, wood splinters and thorns which are much more difficult to stop.
To further improve on the accuracy and repeatability of this test method, multiple needles are used to prevent test bias from an individual needle. Seven (7) needles with two samples per needle are used within a single test and the average of 14 data points are used to represent the puncture resistance of the material. The use of multiple needles adequately compensates for various subtle differences between needles including finish, taper geometry, and sharpness that are non-trivial factors when evaluating protective material puncture resistance fine gage penetrators.
A third preference in optimizing puncture resistance characterization is the procedure used to determine the test end point. A puncture resistant material has failed once the penetrator has broken through the material. As a result, the peak force is not necessarily indicative of the level of protection since the displacement through the material may far exceed the allowable displacement before harm is inflicted on the user. To remedy this discrepancy, an optical test fixture is incorporated into WTM-7N05 to observe when the tip of the penetrator has gone through the material a minimum distance of 0.030″ below the bottom surface of the protective material.
Premium cowhide and deerskin leathers have been used for cut/puncture protection, particularly in gloves. A common thickness used in light duty leather gloves is approximately 0.045 inches (1.1 mm) thick. Such a material has been characterized as having a 7 needle puncture resistance of approximately 1.8 lbs force. The resultant normalized puncture resistance (NPR) rating is 40 lbs/inch of thickness. In more heavy duty industrial applications such as veneer plywood manufacturing, much thicker leather materials are used to protect against severe wood splinter threats. With 10 lbs force resistance in the 0.125″ material, this leather product with an NPR of 96 ppi is seemingly invincible. However, due to its thickness and cut and sew method of assembly, puncture protection is lost at the seams where virtually no protection is provided. With the simplest stitch through this leather product, the puncture resistance rating drops to 6 ppi, resulting in a product has very poor dexterity and ultimately does not provide the necessary protection.
New classes of textile-based materials, under the trademark TURTLESKIN™, have been developed under U.S. Pat. Nos. 5,565,264, and 5,837,623, which are hereby incorporated by reference for all purposes. TURTLESKIN™ brand and similar materials have been incorporated into specialty gloves using many cut and sew glove designs including those similar to designs described in commonly assigned U.S. Pat. Nos. 6,052,829, 6,094,748, and 6,460,192, which are hereby incorporated by reference for all purposes. These glove types provide excellent hand protection, using TURTLESKIN™ brand fabrics.
These materials are produced from a broad range of fiber types including cotton, polyester, aramid (Nylon), meta-aramid (NOMEX®), para-aramid (KEVLAR®, TWARON®), rayon, polybenzimidazole (PBI), polybenzoxazole (PBO), as well as blends of these and other fiber types. Applicant makes no claim to the trademarks NOMEX®, KEVLAR®, and TWARON®. Based on special fiber blends and densely woven constructions, these fabrics have very high resistance to puncture and cut and also tend to be very thin (less than 0.02″, 0.5 mm), providing good flexibility for operations that require tactile sensitivity. In practice, these materials offer a range of puncture protection from NPR of 50 ppi to, or in excess of, 225 ppi, significantly greater than any other polymer, textile, or leather materials commonly used today, particularly in gloves.
When applying the range of TURTLESKIN™ brand puncture resistant materials into protective apparel such as gloves as described by U.S. Pat. Nos. 6,052,829, 6,094,748, and 6,460,192, limitations become apparent due to available cut and sew methods as well costs required to develop a fully custom glove. Because of the wide variety of applications in fields industrial, commercial, civil service, etc. there are nearly infinite combinations of hand and arm protection products used. In many instances, the users have adapted to the feel of existing gloves styles and incorporated that tactility as a control mechanism in the work they perform. Preserving the “feel” and grip provided by the broad range of safety products to the specific applications is highly desirable and hence incorporating these protective materials into existing personal protective equipment product designs without drastically modifying the design of the product is highly desirable.
TABLE 1ThicknessPunctureNPRMaterial typeFiber(in)(lbf)(lbf/in)T9 736#MT1Para-Aramid.0142.3164TurtleSkinstapleT9 1010#MT2Cotton/Para-.0131.077aramid blend1094MT#3Para-Aramid.010.550PalmMasterMT#4Coated Para-.0173.8224AramidCowhideMT#5.0351.646Split cowhideMT#6.523.160MT#1.0455.4>120combined2.3 + 1.6with MT#5MT#2.065.9>98combined1.0 + 3.1with MT#6